Process for producing plastic articles having transparent electroconductive coatings

ABSTRACT

A method for preparing a transparent electrically conductive article which is characterized by forming a coating on the surface of a plastic material by vacuum evaporation using indic oxide as an evaporating source at a pressure lower than 5 X 10 4 mmHg while maintaining the temperature of the evaporating source at 1,300* to 2,000*C., and thereafter subjecting said coating to an oxidation treatment.

United States Patent Sobajima et a1.

[4 1 Nov. 12, 1974 Foreign Application Priority Data Nov. 13, 1971 Japan.., 46-90770 U.S. Cl 1l7/21l, 117/7, 117/62, 117/106 R, 117/213, 204/56 R Int. Cl. HOSb 33/23, C23c 1 1/00 Field of Search 117/211, 106 R, 62, 213, 117/7; 204/56 R References Cited UNITED STATES PATENTS 9 1973 Leibowitz 117/106 R 7 1973 Vossen 117/211 10/1972 Louderback 117/106 R 3,505,092 4/1970 Ryan 1 1 117/106 R 3,458,342 7/1969 Cormia 1 117/106 R 3,400,014 9/1968 Blumberg 1. 117/106 R 3,356,529 12/1967 Kiser ll7/211 OTHER PUBLICATIONS Holland, Vacuum Deposition of Thin Films, John Wiley & Son, N.Y., Pg. X1 (1956).

Primary Examiner-Leon D. Rosdol Assistant E.\'aminer-Michael F. Esposito Attorney, Agent, or Firm-Sherman & Shalloway [57] ABSTRACT A method forpreparing a transparent electrically conductive article which is characterized by forming a coating on the surface of a plastic material by vacuum evaporation using indic oxide as an evaporating source at a pressure lower than 5' X 10 mmHg while maintaining the temperature of the evaporating source at l,300 to 2,000C., and thereafter subjecting said coating to an oxidation treatment.

15 Claims, N0 Drawings PROCESS FOR PRODUCING PLASTIC ARTICLES HAVING TRANSPARENT ELECTROCONDUCTIVE COATINGS under high vacuum on the surface of a shaped plastic article under specified conditions using indic oxide, i.e.

indium (III) oxide, as an evaporating source and then subjecting the article to oxidation. The shaped articles according to the present invention have an excellent transparency, electric conductivity, flexibility, impact strength, processability and continuous producibility, and find utility as transparent electrodes in a solid display and electroluminescence, panel heater, base supports of opto-electronic materials and recording materials, and antistatic plastics.

With the advent of the informationalized society," cathode ray tubes which have been previously used exclusively have been superseded by solid displays such as an electro-luminescence display, liquid crystal display, plasma display or ferroelectric display. Such solid displays require transparent electrodes. Furthermore, opto-electric devices and recording materials utilizing the mutual action and mutual conversion of electrical and optical signals are about to come under the spotlights as a leading player in the information processing technique, spotlight materials having a combination of superior transparency, electroconductivity and flexibility have been demanded for use as component parts of these opto-electric materials. Elsewhere, materials having excellent transparency, electric conductivity and impact strength are desired for use as antifreezing window panes in traffic conveniences such as automobiles or airplanes. On the other hand, the buildup of static charge on shaped plastics often becomes a cause of serious accidents. However, the method of static prevention by incorporation of metal or carbon powders is often not desirable because it involves coloration, opacification and deteriortion of dynamic properties of the shaped plastics. The formation of a transparent electroconductive coating on the surface of the shaped plastics is also excellent as a method of static prevention.

The transparent electrically conducting material which has mainly been used is, for example, NESA- glass, produced by forming a coating of tin oxide on a thermally stable material such asglass or quartz. The materials based on glass or quartz bone defects of flexibility (therefore, cannot be produced continuously), poor processability, low impact strength, heavy weight, and high dielectric-loss. Furthermore, because they cannot be wound up as in the case of magnetic tapes or photographic films, they are difficult to use as-information recording media. Such defects can be almost completely removed by employing polymeric supports (bases) on which transparent electronductive coatings are formed. But as the polymeric materials generally have poor thermal stability, the conventional methods of preparing them are hardly applicable.

The method of producing NESA-glass comprises spraying an aqueous hydrochloric acid solution or alcohol solution of tin chloride onto a glass plate heated at 450 600C. to hydrolyze tin chloride instantaneously on the glass plate. However, plastic materials which are substantially colorless and transparent and have excellent mechanical properties are stable at temperatures below 300C. As examples of a method of forming an electroconductive coating on such plastic materials as polyester films, there can be cited the following:

1. Method comprising depositing indium metal by cathode sputtering or 'vacuum evaporation, followed by heat-treatment of the formed coating of indium metal in air at C. and further treatment with nitric acid, for example, to convert it to in dium oxide.

2. Method comprising vacuum evaporation in oxygen at low pressures using indium metal as an evaporation source or further heat-treating the coating thereby to form a coating of indium oxide.

It is known that the coating of tin oxide or indium oxide are suitable as good transparent electroconductive coatings. Heretofore, metal indium has been used as a starting material in the formation of a coating of indium oxide.

The coating formed by sputtering or vacuum evaporation in the method (1) mentioned above is nontransparent and consists essentially of metallic indium. Since it is considerably difficult to convert metallic indium to its oxide at relatively low temperatures, even when the coating is heat-treated in air under mild temperature conditions which do not affect the base plastics adversely, substantial oxidation of metallic indium is not performed. Accordingly, a transparent coating of indium oxide cannot be obtained. For this reason, this method requires an additional step of treating the coating with nitric acid.

Method (2) mentioned above in which vacuum evaporation is performed in the presence of oxygen at low pressures using metallic indium as an evaporating source is based on the technical concept that a coating of indium oxide is formed by the reaction of the evaporated metallic indium with the oxygen. Since this vacuum evaporation is performed while involving the reaction of metallic indium with oxygen, it is called "reactive vacuum evaporation" or "reactive deposition. This method has the following defects.

Firstly, the rate of forming a coating by vacuum evaporation is low, and the structure of the coated layer is coarse, which results in low strength of bonding to the base material. This is due to the fact that because of the presence of oxygen the mean free path L of the evaporated particles is small. Secondly, it is very difficult in actual operation to perform a continuous vacuum evaporation operation while maintaining a partial oxygen pressure constant. If the partial oxygen pressure fluctuates, the uniformity of the resulting coating is lost. Therefore, from a commercial point of view, it is substantially impossible to form a uniform, stable transparent electroconductive coating on the surface of the base material while continuously moving the base material.

We have noted a report by J. W. Mellor in A Comprehensive Treatise on Inorganic & Theoretical Chemistry describing that freshly prepared indium oxide is more sublimable than indium oxide prepared previously. We have made attempts to form such freshly prepared indium oxide by heating indium metal in a tungsten crucible equipped in a vacuum evaporation apparatus. Since tungsten is oxidized at temperatures higher than 530C. to form sublimable tungsten oxide, the

I tungsten crucible with indium metal in itwas heated to temperatures below 530C. in an atmosphere of circulated oxygen. However, the oxidation was very slow and it was extremely difficult to oxidize indium metal substantially. When the crucible was heated to about 900C. in order to promote the oxidation of indium metal, the indium metal was partially oxidized and, at the same time, the tungsten crucible was highly oxidized. Vacuum evaporation was then carried out at reduced pressure of X mmHg, whereby dull coatings containing a large quantity of tungsten oxide were obtained. Even when the coatings were subsequently heat-treated'in an oxidizing atmosphere, it was substantially impossible to obtain a transparent electroconductive coating.

According to the present invention, a new method free from the various defects of the prior art has now been found.

The process of the present invention comprises forming a coating on the surface of a shaped plastics by vacuum evaporation using indic oxide as an evaporating source at a pressure lower than 5 X 10 mmHg and an evaporant temperature of l,300 to 2,000C., and thereafter subjecting said coating to an oxidation treatment.

The special features and advantages of the new process of the present invention are as follows:

l. A first feature is to use indic oxide as an evaporating source. A greater part of the indie oxide is decomevaporated particles and the distance 1 between the base material and the evaporating source in vacuum evaportion, the rate of deposition is extremely low and the structure of the coating formed becames nonuniform. The mean free path L depends upon the degree of vacuum, and is, for example, about 5 cm at l X 10 mmHg, about [0 cm at 5 X 10 mmHg, about cm at 2 X 10 mmHg, and about 50 cm at l X 10' mmHg. Since the introduction of oxygen is not required in the present invention, lower pressures than 5 X 10' mmHg can be applied. If the pressure is high, the distance I between the evaporating source and the base material must be made smaller in order to maintain the relation L I. For example, since L at l X IO' mmHg is about 5 cm, it is necessary to maintain the distance 1 between the evaporating source and the base material at below 5 cm. As a result, the plastic base is likely to be softened or melted by the heat of irradiation from the source of evaporation. The present invention is free from such likelihood and has the advantage I that a transparent electroconductive coating of good posed in the course of evaporation to indium oxides of the lower oxidation state than indic oxide, and deposits on the surface of a base material to form a coating. The lower oxides of indium may be expressed by ln, O wherein O y/x l .5. The decomposition of indie oxide vacuum-evaporated by being heated to high temperatures to the lower oxides is the same phenomenon as is observed generally in other metal oxides. This will be understood from the fact that the coating formed assumes a gray to black color which is the color of the lower oxides of indium. The lower oxides are electrically conductive but not transparent. These lower oxides can be far more easily oxidizable to indic oxide than metallic indium. In other words, in the vacuum evaporation step followed by the oxidation treatment, coatings comprising the lower oxides are oxidized under far milder conditions and within shorter periods of time than coatings of metallic indium or those containing both metallic indium and oxygen which are formed by the conventional methods. For example, by heating the coating in an oxidizing atmosphere at a relatively low temperature of from 80 to 250C. for a considerably short period of time, the desired transparent coating of indie oxide can be obtained. Thus, the process of the present invention has the advantage that it can be applied to a plastic base and a high productivity is expected.

2. A second feature of the process of the present invention is to perform vacuum evaporation at a pressure lower than 5 X 10 mmHg. Since introduction of oxygen is not necessary as in the conventional reactive deposition process, there is no difficulty of maintaining the oxygen pressure constant but it is possible to maintain the vacuum deposition system at such a low pressure. Low pressures serve to increase the rate of deposition and to form a coating which is uniform and has high adhesion. Generally speaking. if the relation L l is not established between the mean free path L of the quality can be formed at a far higher rate of production than in the conventional methods.

The preferred embodiments of the present invention will be more specifically described below.

The present invention can be applied to almost all synthetic plastics generally available. The conditions employed throughout the entire process of the present invention are mild enough to apply it to plastics, and moreover such conditions can lead to the formation of a transparent electroconductive coating of good quality on the surface of plastics. Typical examples of the plastics are thermoplastic polymers such as polyolefins or vinyl polymer, e.g. polypropylene, poly( methyl methacrylate) or polystyrene, aromatic polyesters such as polyethylene terephthalate, polyethylene naphthalate or polymers derived from bisphenol A and its halides and acid dichlorides, polyamides such as poly-ecaproamide, poly-'y-butyroamide, polyhexamethylene adipamide, poly-m-phenylene isophthalamide or a copolycondensate of m-phenylenediamine and isophthalic acid and terephthalic acid, polyimides, polyamideimides, polybenzimidazoles, and polycarbonates; and thermosetting polymers such as unsaturated polyester resins, epoxy resins, allyl resins, melamine resins, phenol resins, or urea resins. Of these, polyester resins, especially polyethylene naphthalate, are preferred as a base material because of their superior heat resistance, weatherability, transparency and other properties. This will be described in greater detail below. It is desirable that the plastics to be used in this invention should not contain a plasticizer which is easily volatilizable at the reduced pressures to be employed in the vacuum evaporation step in accordance with the present invention.

The form of the shaped plastics may be a film, sheet, plate or blocks produced by injection molding or extrusion molding, etc. Where it is to be used as a transparent electrode for solid displays, the plastics should preferably be in the form of a transparent film or sheet. But when it is used for static prevention or as a panel heater, colored or non-transparent plates or blocks may be used. In this case, too, the transparent indium oxide coating in accordance with the present invention is desirable since it does not impair the color or design of the base plastics. The film or sheet, as referred to above, has a thickness of about 5 microns to about The evaporation source used in the vacuum evaporation step of the present invention is indic oxide. lndic oxide is commercially available, or can be synthesized from indic halides. No example has been known previously-in which a transparent electroconductive coating is formed on the surface of a shaped plastics using indic oxide as an evaporation source. The evaporation source may further contain minor amounts of ingredients which do not hinder the transparency or electric conductivity of the coating finally formed. For example, it may contain less than about 20 percent by weight, based on the total weight of the evaporation source, of tin oxide or lower oxides of indium, in addition to the indic oxide.

The evaporation source must be maintained at a temperature of l,300C. or above.- The upper limit of the temperature of the evaporation source is not particularly set out, but usually 2,000C. At temperatures lower than 1,300C., the rate of evaporation is slow, and it is not commercially advantageous. The especially preferred range of temperature is 1450 to 1,800C.

The following comparative test shows that if the temperature of the evaporation source is lower than 1,3 00C., the rate of vacuum evaporation is very slow.

A coating was formed on a biaxially drawn film of polyethylene terephthalate by vacuum evaporation using 99 percent indic oxide as an evaporation source at 1.2 X 10' mmHg. An alumina-coated tungsten basket was used as a heating element, and the distance 1 between the evaporation source and the base material was maintained at 8 cm (I was made smaller in order to facilitate the measurement of the rate of evaporation at 'low temperatures). The results obtained are tabulated below.

Mean frcc Degree of vacuum path L Rate of (mm Hg) (centimeters) evaporation Crosscut test* 2 X 10 25 5 A/s'ec. 1009; 8 X l0 6.25 0.6 A/scc. 72'7:

The crosscuts test is a test for adherence in which corsscuts (10 X 10) are provided at intervals of 1 mm only on the surface of the indium oxide coating, an adhesive tape is applied onto the crosscut surface and when the tape is removed, the number of remaining crosscuts is measured. This crosscut test was perfomied after subjecting the deposited coating to an oxidation treatment. 7

Since the mean free path L of the evaporated particles increases with lower pressures, it is also possible to maintain the distance I between the plastic base and the evaporation source sufficient for not softening the plastic base by the heat of irradiation from the evaporation source.

The rate of formation of the coating by vacuum evaporation differs according to the temperature of the evaporation source, the degree of pressure reduction and the distance between the evaporation source and Temperature of the (A/sec.)

Heating of the evaporation source may be effected, for example, by electric resistance heating, high frequency induction heating or electron beam heating. If a tungsten filament is used in the electric resistance heating, indium and tungsten form a low melting alloy, and the resulting coating contains tungsten or tungsten oxide which greatly impairs the electric conductivity and. transparency of the coating, or the properties change greatly with the passage of time. The amount of tungsten oxide increases with higher pressures at which the vacuum evaporation is performed. Accordingly, the present invention is performed at very low pressures. Where a tungsten filament is used as a heating element, it is desirable to coat it with alumina or beryllium oxide.

The vacuum evaporation is carried out at a pressure lower than 5 X 10 mmHg (Torn), preferably from 2 X 10' to 1 X 10 Torr. As previously stated, since the introduction of oxygen is not necessary in the present invention, such high vacuum can be applied, and thereby a coating which is uniform and has high adhesion force can be formed at a very high evaporation rate.

the base material. According to the present invention,-

unlike the conventional reactive deposition process involving the reaction between metallic indium and oxygen in the course of evaporation, coatings of similar properties can be formed irrespective of whether the rate of formation of the coating thickness is relatively slow, e.g. 2-5 A./sec or as fast as more than 50 A./sec. Thus, the present electroconductive has the advantage that the process can be performed commercially at high speed.

Thus, there can be formed a coating of any desired thickness at high speed,'such as a coating of a thickness from A to 3,000 A. The most preferred thickness is from 300 to 2,000 A.

If the coating is excessively thin, its electric properties are unstable or the coating has imperfections. On the other hand, if the thickness is too large, it is difficult to provide a transparent coating by subsequent oxidation, especially heating oxidation, of the coating deep into its inside. The thickness of the coating in the above specified range is especially important when the base is a plastic film, because in the above-mentioned range,

the flexibility of the film is not impaired, and the adherence of the coating to the film is great.

The temperature of the base plastics iselevated to some extent during vacuum evaporation by the heat of irradiation from the evaporation source and the heating element. For example, in electric resistance heating or high frequency induction heating, the temperature of the base plastics may reach 150C. or higher. In the case of electron beam heating, the temperature of the base material can be maintained at about 60C. Furthermore, by forcibly cooling or heating the base material, the temperature can be suitably controlled. The temperature of the base material should of course be adjusted to below the heat-resistance temperature of the shaped plastics. By the heat-resistance temperature -is meant the maximum temperature at which the 'thalate film, 130- 140C. for a plate of polymethyl methacrylate and 140 160C. for a shaped article of polycarbonate. The heat-resistance temperature can be known simply by setting a test piece under a constant length, heating it in a constant temperature vessel in this state at a rate of 2C./min. and detect the maximum temperature at which there is visually no formation of creases and cracks or extreme coloration.

In order to improve the adhesion between the resulting coating and the plastics base, a primer may be formed on the surface of the base material. The primer should be chosen so that it should not leave a volatile material in order to achieve a high degree of vacuum in the step of vacuum evaporation, it should have heat resistance at temperature used in the oxidation treatment, and it should have transparency which does not impair the properties of the transparent electrically conductive coating. For example, a coating agent comprising a silicone resin or epoxy resin is a preferred primer material.

It is desirable that the surface of the shaped plastics should be washed before the coating of a primer. The washing of the surface may be carried out, for example, by ultrasonic vibration in an organic solvent such as carbon tetrachloride or acetone. It is also possible to impart ion bombardment to shaped plastics, or treat it with a mixed bichromic acid solution or with an oxidizing flame.

The vacuum evaporating apparatus may be of the bell jar type for laboratory use, but forcommercial practice, a continuous vacuum evaporating apparatus or an apparatus including a rotary sample stand is preferred. Especially when the base material can be wound up in roll form, such as films or sheets, it is convenient to use a continuous apparatus in which a feed roll and atake-up roll are provided in a vacuum chamber.

The coating formed on the surface of the base material by the vacuum evaporation process in accordance ides of indium formed during the process of evaporation of indic oxide. The oxides are converted to the desired transparent indic oxide by oxidation treatment.

under mild conditions. By the oxidation treatment," as used in the present specification and claims is meant a treatment by which a coating consisting mainly of the lower oxides of indium formed on the surface of the base material by vacuum evaporation is subjected to oxidizing conditions to convert the lower oxides substantially to indic oxide. It has been found that the oxidation treatment may be carried out by heating the coating in an oxidizing atmosphere, subjecting the coating to anodic oxidation, or contacting the coating with an aqueous solution containing an oxidant. These methods will be described below in greater detail.

First the method of heating the coating in an oxidizing atmosphere will be described with reference to a preferred embodiment.

The oxidizing atmosphere is an atmosphere required for oxidation which is air or oxygen gas or one obtained by activating air or oxygen by ultraviolet ray radiation or discharge. Those containing at least 15 percent by volume of oxygen are preferred. Generally. with increasing amounts of oxygen gas and with increasing degree of activation, the temperature necessary for oxidation may be lower and, the time may be shorter.

It has been found as a result of experiments of the inventors that the relation between the heating temperature T (K) in air at 1 atmosphere required to increase the transmission of the light of 600 nm (nanometers) through a coating of the lower oxides of indium having a thickness of 800 A. to at least 80 percent and the treating time 1 (minutes) is-expressed by the following empirical formula:

t= 3.0 X '10- e 1.65 X Wherein k is the Boltzmanns constant gree],

e is the base of natural logarithm, and T is 400 550 K. This means that with higher temperatures, the time required becomes shorter. From this equation, it can be seen that the oxidation treatment may be carried out, for example, at 200C. for eyto inut s;

If the atmosphere is pure oxygen, the conditions are much milder, and for example, the oxidation treatment can be carried out at 150C. for 20 minutes.

In order to carry out the oxidation treatment at a feasible velocity, it is preferred to employ heating temperatures of above about C. Of course, the heating temperature should be lower than the heat-resistance temperature of the base plastics. Since the heat-resistance differs according to the type of the plastics used, it can be elevated depending upon the base material to be used. Suitable oxidizing conditions for several plastics are determined as follows in consideration of the heatresistance temperature of the base material and the rate of the oxidation treatment.

According to the present invention, the maximum temperature during the production process can be adjusted to below 250C. and below 200C. by the deposition step in high vacuum and the oxidation treatment in an oxidizing atmosphere. By a combination of these steps, it has become possible to produce a transparent electroconductive coating of very desirable properties at a high rate of evaporation. According to the process of the present invention, a transparent electroconductive coating having a specific resistance of 10 to 10" [1.3 X 10 erg/de- Heat treatment Heat treatment ohm.cm and a transmission of 600 nm light of 60 95 percent can easily formed on the surface of shaped plastics.

Another embodiment of the oxidation treatment comprises subjecting the coating formed by vacuum evaporation to anodic oxidation. The anodic oxidation technique itself is well known, and is not considered to require any detailed description. Briefly stated, the anodic oxidation is carried out using an electrically conducting liquid medium in an electrolytic cell in which the coating formed by vacuum evaporation is used as an anode and a metal such as copper, aluminum, iron,

tantalum, platinum or graphite is used as a cathode, and electrode potential is applied between the electrodes to an extent such that the lower oxides of indium which constitute the coating are oxidized. By the control of electricity being consumed at the anode, the coating changes into a transparent electronductive coating, and there can be obtained a shaped plastic having a coating with the desired electric conductivity and transparency.

The liquid medium used for the anodic oxidation in accordance with this invention may be any medium which advances anodic oxidation, but those having a specific conductivity of ohm'hcm to 10 oh-- m. cm", especially 10 ohm.cm" to 10 ohm cn f are preferred lixarnples are as follows:

1. water, 2. an aqueous solution of an inorganic acid such as hydrochloric acid, sulfuric acid, nitric acid, boricacid or phosphoric acid, preferably sulfuric acid, nitric acid and boric acid,

3. an aqueous solution of an organic acid such as acetic acid, oxalic acid, tartaric acid, citric acid or succinic acid, preferably tartaric acid and succinic acid,

4. an aqueous solution of an inorganic or organic acid salt, preferably ammonium borate, potassium hydrogen, sulfate, ammonium sulfate and sodium tartrate,

5. alcohols such as methanol, ethanol or glycerol, phenols such as phenol, naphthol, hydroguinone or anthraquinone, ketones such as acetone, methyl ethyl ketone or methyl isobutyl ketone, esters such as ethyl acetate, ethyl propionate or ethyl butyrate, ethers such as dimethyl ether, diethyl ether or methyl ethyl ether, amides such as dimethyl formamide, dimethyl acetamide, pyrrolidone or N- methylpyrrolidone, nitriles such as acetonitrile, propionitrile or benzonitrile, nitro compounds such as nitrobenzene or nitronaphthalene, dimethyl sulfoxide, butyrolactone; preferably methanol, butyrolactone, acetonitrile, dimethyl formamide and dimethyl sulfoxide, and

6. an aqueous solution of the organic compound listed in (5), preferably methanol, butyrolactone,

acetonitrile, dimethyl formamide or dimethyl sulfoxide.

These media can be used in mixture, and especially dimethyl formamide and dimethyl sulfoxide have proved to be very suitable for use in the present invention.

This anodic oxidation method is advantageous in that it can be performed continuously at room temperature, and can be applied to plastics having a relatively low thermal stability. Since this method can be performed under milder conditions than in the conventional method, a uniform product can be obtained easily. By controlling the amount of electricity required for anodic oxidation, there can be produced a coating of the desired electric conductivity and transparency, and the properties of the coating can thus be automatically controlled.

Another embodiment of the oxidation treatment involves contacting the coating with a solution containing an oxidizing agent. Since the coating consists essentially of the lower oxides of indium, they can be easily oxidized to indic oxide, unlike metallic indium.

The type of the oxidizing agent to be used is not particularly restricted, and customary oxidizing inorganic chemicals are preferably used. Specific examples include permanganic acid salts, bichromic acid and its salts, persulfuric acid and its salts, hydrohalic acids and their salts, perhydrohalic acids andtheir salts, hydrogen peroxide, cerium sulfate, and ferric chloride. Especially, permanganates, bichromates and persulfates are suitable.

The oxidizing agent is used as a solution in water, acetic acid, dimethyl formamide, etc. The oxidizing power of the oxidizing agent may be increased by heating the solution at the time of oxidation, or in the case of permanganates or bichromates, by adding an acid such as dilute sulfuric acid or an alkali such as sodium hydroxide. The concentration of the oxidizing agent in the solution differs according to the type of the oxidizing agent, and in the case of permanganates or bichromates for example, it is 2'- 3 percent by weight. In the case of persulfates, the concentration is about 10 percent. The suitable concentration for aqueous hydrogen peroxide is about 30 percent. Where the oxidizing action of the oxidizing agent is strengthened by adding a dilute sulfuric acid or sodium hydroxide, it is preferred to adjust the amount of the acid or alkali so that the pH of the solution becomes 1 13, preferably 2 12.

The shaped plastics on which surface is formed a coating of the lower oxides of indium is immersed in a solution containing the oxidizing agent to oxidize the coating chemically. It is sufficient that the immersion time is several seconds at room temperature when a solution of potassium permanganate or potassium bichromate acidified with sulfuric acid is employed. When an aqueous solution of persulfuric acid is used, the immersion time should be about 30 minutes.

The oxidation treatment using the oxidizing agentcontaining solution can be performed at room tempera ture, or if desired, by mild heating. Therefore,'the heat resistance of the plastic base need not to be considered in performing the oxidation treatment. Furthermore, since this oxidation treatment is based on chemical oxidation with an oxidizing agent under mild conditions, a uniform, transparent electroconductive coating can be produced with a very simple operation. Such a method has not been known previously.

By the vacuum evaporation step and the oxidation treatment step described above, a transparent electroconductive coating can be formed on the surface of shaped plastics. However, in certain cases, it is desired to improve the transparency of the coating further. For example, when the thickness of the coating is large and the effect of the oxidation treatment does not extend to the interior of the coated layer, or when oxidation is not sufficiently performed in order to shorten the oxidation treatment time or to make the conditions mild, or when there is a local fluctuation in the thickness of the coating due to vacuum evaporation on to a base material having a wide area, the oxidized coating might have insufficient transparency or non-uniform trans parency. The insufficient transparency is attributed to the fact that some amounts of the lower oxides of indium still exist. A new method of improving the transparency of the coating further in such a case has now been discovered, which involves chlorinating the coating, This is based on the fact that the reactivity for chlorinatin of indie oxides differs from that of the lower oxides of indium, and while the former is difficult to chlorinate, the latter is easily chlorinated to a water-soluble chloride and can be removed from the coating.

The chlorinating agent may be gaseous or liquid, and is not particularly restricted. Usually, chlorine, hydrogen chloride, nitrogen oxychloride, hydrochloric acid, and diluted aqua regia are used. Diluted hydrochloric acid is especially preferred-The chlorinating temperature is room temperature and if desireda slightly elevated temperature. The time required for chlorination differs according to the type and concentration of the chlorinating agent, the chlorinating temperature and the desired degree of transparency, but usually is about 3 seconds to about 30 minutes. The treated coating is washed with a solvent capable of dissolving the chloride, usually water, and dried.

As described above, the chlorination operation is very simple, and by performing this step after the oxidation treatment, it is possible to improve the transparency of the coating and the uniformity of the transparency. Hence, even when the oxidation is insufficient in the oxidation treatment step, there can be obtained a product having excellent transparency by performing this chlorination. In other words, by performing the chlorination treatment, the oxidation time required for obtaining transparency of the same level can be remarkably shortened, for example, to about half. 7

Thepla s ties having a transparent electroconductive coating in accordance with this invention are electrically conductive while retaining the properties of the plastics, and unlike the conventional electroconductive rubbers or plastics containing the fine powders of metal or carbon, the coating can be made colorless and transparent. Therefore, the shaped plastics according to the present invention find a wide range of utility, for example, as transparent electrodes in solid displays for use I 12 as indicator plates of portable electronic computers or digital wrist watches, transparent electrodes for use in panel illumination, anti-freezing window panes laminated on glass, etc. in traffic facilities, plates for reflecting thermic rays, or electrodes for use in recording materials utilizing the mutual action of light and electricity such as electrophotographic recording materials, electrolytic recording materials or electron beam recording materials. For example, the shaped plastics should be in the form of a film when used as recording films, and also, they should be transparent and have excellent physical properties. Generally. polyimides have excellent thermal stability and dimensional stability, but inherently have the defect that they are colored and have a water absorbing ratio of 2 5 percent. Especially. the

coloration is a fatal defect as a transparent electroconductive film. From this viewpoint, a polyester such as polyethylene terephthalate is preferably used as a base of a transparent electroconductive film.

Polyethylene terephthalate has transparency over a wide visible region and superior mechanical properties and dimensional stability at room tempeatures, and because of this, it is an excellent material as a base support of a transparent electroconductive film. However, it is still required to be improved in regard to dimensional stability at higher temperatures and resistance to heat degradation and to ultraviolet degradation. It has' now been found that a transparent electroconductive film of outstanding quality can be provided by forming a coating of indie oxide on a film of polyethylene 2,6- naph thalenedicarboxylate. The polyethylene-2,6 naphthalenedicarboxylate is a polyester in which at least mol percent of the recurring units is a polymer obtained by polycondensation of ethylene glycol and naphthalene-2,6-dicarboxylic acid or substantially the same polymer as this. The remainder of the recurring units is derived from another component copolymerized with the polymer. The other component may, for example, be an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid, phthalic acid, naphthalene-2,7-dicarboxylic acid, or diphenyldicarboxylic acid and/or a diol such as propylene glycol, trimethylene glycol, tetramethylene glycol, neopentyl glycol, p-xylene glycol, 1,4-cyclohexane dimethanol or hydrated bisphenol A. The polyethylene-2,6- naphthalenedicarboxylate may also be a blend consisting of at least 80 mol percent of a polymer obtained by polycondensation of ethylene glycol and naphthalene- 2,6-dicarboxylic acid and the remainder of at least one polymer obtained by polycondensation of the abovementioned dicarboxylic acid and glycol components. If desired, the polyester may contain a stabilizer such as phosphoric acid, phosphorous acid or their esters, or an additive such as finely divided silica or China clay.

It is especially preferred that this film be a biaxially stretched film. The biaxially stretched film is one obtained by stretching an unstretched film formed by melt shaping in at least two directions, usually in perpendicular directions (to be referred to as longitudinal stretching and transverse stretching). The film for use as a base material in the present invention maybe any filmwhich is stretched substantially both in the longitudinal and transverse directions, irrespective of whether the stretching is performed simultaneously in two directions, successively in two directions, or by a combination of these procedures. The stretch ratio is not particularly limited. In order not to cause unevenness in 13 thickness due to the non-uniformity of stretching, the draw ratio in each of the transverse and longitudinal directions is at leat 2, and it is preferred that either longitudinally or transversely, the stretch ratio should exceed 2.5. By so doing (and by choosing a proper stretching temperature), it is possible to restrain the thickness unevenness within percent.

The superiority of a coated film structure comprising a base film of polyethylene-2,6- naphthalenedicarboxylate (to be abbreviated as PEN) and coating of indic oxide will be described below in general terms with reference to some illustrative showings.

a. Transparency A transparent electrically conductive film having a coating of indic oxide formed on a base support of PEN exhibits good transparency expressed by a light transmission of to 90 percent (including the base) in a visible region. Such a good transparency is due to the formation of the transparent electroconductive film in accordance with the present invention. The PEN film base has good transparency in a visible region same as a polyethylene terephthalate film, and has a superior transparency to a polyimide film as shown in the following table.

Light transmission Wavelengths of 50 ptm-thick Light transmission of light PEN film of 50 am-thick (nm) (7%) polyimide film formed thereon, distortion hardly occurs between the base and the coating.

c. Thermal stability Since PEN has a glass transition temperature of C., about 50C. higher than that of PET, and also has high initial Youngs modulus at high temperatures, it has superior thermal dimensional stability. For example, the biaxially stretched PEN film has .a dry heat shrinkage in a free condition at C. ofnot more than 1.0 percent, but the dry heat shrinkage of the biaxially stretched PET film under the same conditions is as high as 6.0 percent. Such a difference in thermal di' mensional stability appears as a great difference in the area to which the film can be applied. According to the thermal stability classification in the electric industry, materials comprising PET film are of E grade, while those comprising PEN film are of F grade.

In addition to the stability as used at the temperatures described above, the resistance of the film to a soldering bath used at the time of connecting the film to an electric circuit also poses a problem. The PET film often shrinks considerably in a soldering bath at below 225C, and becomes useless. PEN films of thermally stable grade having a high density (at least 1.355) which is obtained by heat-treating the stretched film at below 250C. and then again heat-treated at above 260C. can be soldered using a soldering bath at a temperature as high as 250C.

The thermal stability of the base material determines also the easiness of the production of the transparent electroconductive film. As previously stated, when the coating formed by vacuum evaporation is heated in an oxidizing atmosphere, the heating time may be shorter as the heating temperature is higher, and a film of better transparency can be obtained. PEN can withstand such high temperatures, and therefore, transparent electroconductive films of better properties can be obtained at lower cost.

d. weatherability In applications where the transparent electroconductive film is used outdoors (for example, when used as a solar battery or a photoswitch), the weatherability of MD: machine direction, TD: transverse direction Balanced film: the stretch ratios in both direction are substantially the same As is seen from the above table, the biaxially stretched film of PEN has better mechanical properties than the PET film which is said to have excellent dimensional stability. It especially has high initial Youngs modulus, and when a coating of indic oxide is the film becomes a problem. When a PEN film and a PET film were respectively exposed to the irradiation of a Xenon lamp for 200 hours, and the ratio of deterioration of strength at breakage was measured before and after the irradiation, it was found that the former deteriora ted by Sperc'en t whereas thelatter deteriorated bySO percent. y

- The transparent electroconductive PEN film is a' novel film having a balanced high level of mechanical;

I ;electric'al,optical and thermal properties. If only the ithermal properties, for example, are considered, trans- I parent electroconductive films based on polyimide films are excellent, and if the optical properties alone are considered, those based on cellulose triacetate films are excellent. However, the transparent electroconductive films basedon PEN have a balanced outpresent invention;

The following examples are given to illustrate the 7 EXAMPLE 1 i Ad-8 m thick" film" 'orl polyethylene-2,6-

(specific resistance'of 2.0 X 10 ohmsctn). This film was lighter in weight than the conventional Nesaglass, had goodflexibility and processability (cutting,'punching, etc.), and lent itself to easy handling.

g EXAMPPEZI...

and the temperature of the heating element at l,500C.

There was formed a black coating of-the lower oxides of indium having a thickness of about 650 A. The filmhaving the coatingwas heat-treated in air at 180C,

200C, and 230C, respectively'The results are shown i Ta le 3 be w;

Table 3 1 Tem 7 180C 1 .20oc 230C Pro'per- Trans- Surface Trans- 7 Surface Tr'ans Surface ties' mitlance resistmittance resist mittance resist Time at 600'nm ance at 600 nmance 211600 nm ance. l (minutes) (ohmslcm ('72) (ohms/cm) -ohm s/c m 0. 31' 730 720. 3r I I 730 (before wheat treatment) 5' 62 220 560 7 89 960 15 (no 82 760B" .92 i200 30-. 750 87 900 6O 86 '880 1 90 H0O I20 '89, g900 heat-treated'atZ BOfC. (th e film had been pretreatedby A LE 3 -immersingin a mixed;bichromic acid"solution, washing it sufficiently with'water and drying it) was set under constant length in a vacuum evaporation apparatus I with the distance between an :evaporationsource and V v t'he base film beingkept at 30 cm ,'and 99 percent'indic oxide was set in an alumina-coated tungsten crucible, gin; which was heated-at about -l',600 C", thenNacuum evaporation was performed at a rate of 6 A./sec. under "a'pressure of 1 X 10 mmHg. f

The-resulting coating consisted of the lower oxides of I indium. jltwas black in color and had a thickness of 800 A.,-a-su'rface resistanceof 3.3O-ohrns/cm and a trans-v .mission of light'of 600 nmof 20 percent.

' I When the filmhaving the coating of lower oxides of 'indium'washeat-treated at250C. in air, the transparency of the coating increased with time, and when 10 I minutes passed, the transmittance of the coating in a wavelength region from 400 nm to 700 nm reached 85 9.2 percent.( The transmittance of light was measured by setting. the film before vacuum evaporation in a compensationlight-path.

' The surface resistance of the coating gradually decreased forthefirst '1 to 3 minutes, butincrease after alapseof3 minutes. When-the'film was heat-tre ted in hair at 250C. for 5 minutes (theconditions chosen in consideration of the transparency and the electric con- I du'ctivit'y), there was obtained a transparent electrically conductive coating having a transmittance at 600. nm 12(8 r stttand asur sis s f 5.0 Ohms/m2 100 uthick polyethylene terephthalate film which had been biaxially stretched and heat-treated-at 210C.

was pre-treated in the same was as in'Example l, and

fixed at under constant length to a water-cooled cold 'plate, and 99 percent indic 'oxide was vacuumevaporated by the same apparatus as used in Example 1'. The degree of vacuum was 6 X 10 mmHg; the temperature of the heating element was about 1,450C.; and the rate of vacuum evaporation was 2 A./sec. The distance between the evaporating source and the base material was maintained at 350m. The resulting coat- EXAMPLE 4 The same polyethylene terephthalate film as used'in Example 3 'was subjected to vacuum evaporation under the same conditions as mentioned above to form a black film having deposited thereon a coating of the lower oxides-of indium. This film was heat-treated in air and in oxygen gas respectively under the followingconditions. The results obtainedareshown in Table 4.

Table 4 Atmosphere In oxygen gas l mm.) In air Temp. 150C. l80C. 210C.

lfroper- Trans- Surface Trans- Surface Trans- Surface ties mittance resistmittance resistmittance resist Time at 600 nm ance at 600 nm ance at 600 nm ance (minutes) (71) (ohms/cm") (72) (ohms/cm) (7c) tohmslcm 24 500 25 l0 24 500 (before heat' treatment) 5 32 420 60 560 32 430 l5v 4i 380 70 650 52 400 30 '48 360 77 850 65 360 60 58 420 83 I200 85 470 I80 83 490 EXAMPLE 5 EXAMPLE 7 A 3 mm-thick sheet of poly(diallyl isophthala'te) was washed by ultrasonic vibration in an acetone bath to make a base. Using the same apparatus as used in Example l the base sheet was subjected to vacuum evaporation at a pressure of 5 X 10 mmHg and 99 percent indic oxide was evaporated at about 1,700C. (a rate of 1.4 A./sec.) The distance between the evaporating source and the base was maintained at cm. There was obtained a black coating having a thickness of about 1,250 A., a surface resistance of 420 ohms/cm and a transmittance at 600 nm of about 17 percent. The sheet having the coating was heat-treated at 230C. for 30 minutes in air to form a transparent electroconductive coating having a transmittance of 80 percent and a surface resistance of 210 ohms/cm (specific resistance of 2.6 X 10" ohm.cm).

EXAMPLE 6 Indie oxide powders containing 5 percent by weight of stannic oxide were formed into tablets at 100 Kg/cm and 600C. over 5 hours'to form a vacuum evaporating sample. The tablets were deposited at l X 10' mmHg on a polyethylene naphthalate film at a rate of about 13 A./sec. using an electron gun (model E, product of Nippon Electric Varian Co.). The distance between theevaporating source and the base material was maintained at 25 cm. There was obtained a black coating having a thickness of about 800 A., a surface resistanceof 1,300 ohms/cm and transmittance of 600 nm of 19 percent. This film was heat-treated for 40 minutes at 200C. in air to form a transparent electroconductive film having a coating with a surface resistance of 190 ohms/cm and a percent light transmission at 600 nm of 90 percent.

Vacuum evaporation wa's performed under the same conditions as in Example 6 using the same evaporating source, basefilm and apparatus as used in Example 6.

except that the content of the stannic oxide was 10 percent by weight, the distance between the evaporating source and the base material and the degree of vacuum and the rate of vacuum evaporation were maintained respectively at 15 cm, 9 X l0 and 42 A./sec. There was obtained a blackcozitirig having a thickness of about 550 A.,' a surface resistance of 1,700 ohms/cm and a transmittance at 600 nm of 2 l percent. The film having the aming so formed was heat-treated in air at 210C. for 30 minutes to provide a transparent electroc'onductive coating having a transmittance at 600 nm of 94 percent and a surface resistance of 430 ohms/cm? This means that even when the rate of vacuum evaporation is considerably high, substantially the same coating as in the case of low rates of vacuum evaporation can be obtained.

EXAMPLE 8 Vacuum evaporation was performed by substantially the same procedure as in Example 7 except that the degree of vacuum was about 5 X 10 mmHg and the rate of vacuum evaporation was about 12 A./sec. The temperature of the base film was maintained at C.. 130C. and 200C., respectively. There was formed a coating having a thickness of about 500 A. The film having the coating so formed was heat-treated in air at 200C. for 30 minutes, and the results obtained are shown in Table 5 below.

Table 5 Temp. of I I the film 60C 130C 200C Prope'r- Trans- Surface Trans Surface Tr-ans Surface ties mittance resistmittance resistmitt'ance resistat 600 nm ance at 600 nm arise at 600 nm ancc (72) (ohmslcm 7:) (ohms/cm (71) (ohms/cm") Before heat- 20 2100 37 720 52 M0 treatment After heat' 91 420 360 93 330 treatment EXAMPLE 9 small amount of electricity (this means that the time required for the oxidation is-short). Therefore, it is a new method for producing the transparent electroconductive coating.

A 50 pm thick polyethylene naphthalate film was biaxially stretched and heat-treated at 230C. The film was set in a vacuum evaporating appratus under constant length. At a pressure of l X 10' mmHg, 99 per- EXAMPLE cent indie oxide placed in a crucible made of beryllium oxide was heated to about 1,600"C. by a tungsten filament and deposited on the film which was 30 cm apart from the evaporating source. In

polypropylene. polystyrene and poly(methyl metha- The resultmg coatmg conslsted of the lower Oxldes of crylate). The distance between the base sheet and the indium, which was black in color and had a thickness evaporating Source was maintained at 2 Cm 99 percent indic oxide was vacuum deposited by an electron gun at a pressure of 2 X mmHg on the surface of a base sheet 3 mm thick of polycarbonate,

1,200 a Surface resistance of 450 Ohms/3mg and The resulting coating consisted of the lower oxides of transmittance at 600 of about 20 Percent indium, and there was obtained a black electroconduc- Using this film having black electroconducttve Coat tive sheet having a coating thickness of 1,500 A.. a surtng as an anode and a'platinum plate as a Cathode, the face resistance of 300 ohms/cm and a percent light coating of the lower oxides of indium was subjected to transmission at 600 nm of about 15 percent,

anodic oxidation in various organic solvents at room Th Coating f h lower id f i di was temperature as an electrolyte Under the Varlous elecjected to anodic oxidation under the various electror ly lc CO O s Shown n Table 6- h results are lytic conditions shown in Table 7 using the black elec- PQ Table troconductive sheet as an anode and a tantalum metal Table 6 Anodic oxidation Characteristics of the conditions transparent electroconductive coating I Amount of Solvent Voltage of Current electri- Surface Transthe cell density city used resistance mittance* (Volt) at the for (ohmslcm (71) anode oxidation (mA/cm (coulomb Dimethyl I sulfoxide 160 0.55 0.03 1,000 85 92 Dimethyl formamide 100 0.50 0.04 2,500 85 92 Acetonitrile 60 0.55 0.04 15,000 85 92 Methanol 70 0.50 004 l5,000 70 85 The transmittance in a wavelength region of 400 nm to 700 nm by placing the film before vacuum evaporation in a compensation light path.

As shown in Table 6, the electrolytic oxidation in acplate as a cathode. There was obtained a shaped article cordance with the present invention is carried out at having the trnsparent electroconductive coating deroom temperature at a low current density and with a scribed in Table 7.

Table 7 Properties of Anodic oxidation conditions the transparent electroconductivc coating Base Amount of Voltage Current elect Surface Trans- Electof the density ricity resistance mittancc rolyte cell at the used for (ohmslcm (71) (volts) anode oxidation (mA/cm (coulomb/ Aqueous solution Polyof sulfu- 2.6 1.0 3.0 20.000 85 92 carbonate ric acid (pH=4.8) Aqueous solution 7 Polyof nitric 2.0 0.1 0.04 3,500 70 80 styrene acid (pH=2.5) Aqueous Polysolution (methyl of boric 6.5 0.7 0.6 5.000 methacid acrylatc) (0.05?! by weight) Aqueous solution Polyof tarta- 2.0 0.5 0.03 L300 6t) 70 propylene ric acid weight) milder conditions than in the heat decomposition m e thod athigh temperatures. Accordingly, this anodic EXAMPLE 12 Indie oxide was vacuum deposited on the surface of a 3 mm thick sheet of polycarbonate, polypropylene. poly(methyl. methacr'ylate) and polystyrene under the same conditions as in Example to form a coating having similar properties.

The sheet having the black electroconductive coating so formed was immersed in an aqeuous solution containing the various oxidizing agents indicated in Table '9 to chemically oxidize the coating of the lower oxides of indium. The properties of the resulting coatings are also shown in Table 9.

Table 9 Properties of Chemical oxidation conditions the transparent vention is a new method of producing a transparent electroconductive coating of excellent uniformity unde'r automatic control.

' EMPt A Table 8 electroconductive cogtL Base Species and con- Tempera- Oxidiz- Surface Transcentration of the ture ing resistmittance oxidizing agent (C) time ance (ohms/em (71) Aqueous solution of Polypotassium manganate 25 10 900 70-75 carbonate acidified with H SO seconds (1.6 wt.7( (pH=2.7) Poly- Aqueous solution of propylene persulfuric acid 3.5 3,500 7580 I (10 wt.7c) minutes Poly Aqueous solution of (methyl potassium bichromate 25 5 2,500 75-80 me (1.0 wt.%) minutes acrylate) Aqueous solution of Poly potassium permanganate 25 3 1,200 -80 styrene (1.6 wtf/r) minutes oxidation method 'in accordance with the present m- 35 It 18 clearly seen from Table 9 that according to the chemical oxidation method in accordance with the present invention, a transparent electroconductive coating can be formed on the surface of ordinary versatile plastics of relatively low heat resistance temperature.

For comparison, a coating of indium metal with a thickness of 1,000 A., was formed by vacuum evaporation on a film of polyethylene terephthalate, and without further treatment, the film was immersed in an aqueous solution of the oxidizing agent mentioned above. The surface was dissolved partly, and became an electroconductive film which was apparently transparent with considerable non-uniformity in transparency.

Properties of the transparent It is seen from Table 8 that since the chemical oxidation method in accordance with the present invention can be performed at room temperature, it can be fully pizli dtapla ticwf 9 h ta H EXAMPLE 13 Indie oxide was vacuum evaporated and deposited on the surface of a 50 pm thick polyethylene naphthalate film which had been biaxially stretched and heattreated at 230C., in the same way as in Example 9. Similarly, indic oxide containing 5 percent of. tin oxide 'was vacuum evaporated. In either case, there was obtained a black electroconductive coating of the lower oxides having a thickness of about 2,000 A to 3,000 A. 5

The film containing the coating so formed was heattreated under constant length in air at 200C. for as form a black coating of the lower oxides of indium having a thickness of about 500 A. The film having the coating so formed was-heat-treated in air at 200C. under constant length. The relation between the treating time and the properties of the treated coating was determined, and shown in Table 12.

. Table 12 long as 1.5 hours. There was obtamed a coating of insufficlent transparency as shown in Table 10. Heattreming time 7 lo 15 20 The semi-transparent electroconductive coating so (minutes) obtained was then exposed to hydrogen chloride, chlorine gas, and vapor of aqua regia respectively, washed (ohms/amt) 960 040 1250 1350 500 withwater and dried.'As a result, there could be obtained a transparent electroconductive coating having excellent transparency as shown in Table 10.

When the film having the coating was heat-treated for 10 minutes, 15 minutes and minutes respec- Table 10 Coating of the lower oxides of indium containing a tiny amount of tin Before chlorination Coating of the lower oxides .of indium Treating Before chlorination After chlorination After chlorination agent Surface Trans Treating Surface Trans- Surface Trans- Treating Surface Transresistance mittance* time resistance mittance resistance mittance time resistance mittance (ohmslcm (ohms/cm (7r) (ohms/cm (71) (ohms/cm) (72) Hydrogen 450 88 11 L6 k 98 135 68 l 630 94 chloride seconds minutes Chlorine 300 57 10 2.8 k 95 gas minutes Vapor of I20 48 13 3 k 92 120 65 v57 660 93 aqua regia seconds seconds Trans mittance for light having a wavelength of 600 nm As is seen from the table, the chlorination treatment was performed within very short periodsof time, and

especially when tin is added, there was obtained a coating having excellent transparency and electric conductively, and then exposed to hydrogen chloride gas for about 10 seconds, followed by washing with water and drying, there was obtained a coating having the properties shown in Table 13.

tivity. Table 13 v EXAMPLE l4 Heat-treatin time .The coating obtamed m the same way as in Example (minutes) g 10 15 20 9 was immersed in an aqueous hydrochloric acid solution having a concentration of 2, l0 and 35 percent re- 3 spectively, and there was obtained a transparent elecg g n 2) 94 94.5 97 troconductive coating of sufficiently improved transg zggf sff 2200 1900 I600 parency as shown in Table l 1 below. In this case also,

It is clear from Table 13 that even when the coating is relatively thin, its transparency can be greatly improved without significantly impairing its surface resistance by carrying out the heat-oxidation for the same the chlorination treatment was performed within relatively short periods of time, and by choosing a suitable concentration, there can be obtained a transparent electroconductive coating of superior properties.

Table l l Coating of the lower oxide of indium Coating of the lower oxides of indium containing a tiny amount of tin Treating Before chlorination After chlorination Before chlorination After chlorination agent Surface Trans Treating Surface Trans- Surface Trans Treating Surface Transresistance mittance time resistance mittance resistance mittance time resistance mittance (ohms/cm) (72) (ohms/cm (7:) (ohms/cm (71) (ohms/em (71) 2% hydro- 165 34 4 8.7 k 90 I00 20 3.4 k

chloric acid minutes minutes ,l0% hydro- 140 33 20' I00 k 70, 55 1.7- 550 k 90 chloric acid 7 seconds seconds 35% hydro- 340 61 I9 3 k 90 215 73 4.l k 95' chloric acid seconds seconds EXAMPLE 15 period of time. When the heat-treating time is short- H lndicoxide was vacuum evaporated and deposited'on the surface of a 75 mm thick polyethylene terephthalate film under the same conditions as inExample l to ened to half, the transparency increases and the surface resistance increases by only 700 ohms. From this, it can be concluded that the process of the present invention exhibits great effects in shortening the heat-treating time.

What we claim is:

1. A method for' preparing a transparent electrically conductive article which comprises forming a gray to black color coating of lower oxides of indium on the surface of a plastic material by vacuum evaporation using indic oxide as an evaporating source at a pressure lower than 5 X 1O mmHg while maintaining the temperature of the evaporating source at 1,300 to 2,000C., and thereafter oxidizing said coating until the coating becomes transparent so that the transmittance of 600 nonameters light through the coating reaches 60 to 95 percent.

2. The method of claim 1 wherein the temperature of the evaporating source is 1,450 to 1,800C.

3. The method of claim 1 wherein said oxidation treatment comprises heating the coating formed by vacuum evaporation in an atmosphere of air or a gas containing at least 15 percent by volume of oxygen at a temperature of 80 to 250C. but lower than the heatresistance temperature of the plastic material.

4. The method of claim 3 wherein the coating has a thickness of 300 to 2000 A.

5. The method of claim 1 wherein said oxidation treatment comprises subjecting the coating formed by vacuum evaporation to anodic oxidation in a liquid medium having a specific conductivity of from 10 ohm cm to 10' ohm cm".

6. The method of claim 5 wherein the liquid medium has a specific conductivity of from ohm cm to 10 ohm* cm.

7. The method of claim 6 wherein the liquid medium is dimethyl sulfoxide o dimethyl formamide.

8. The method of 'claim 1 wherein said oxidation treatment comprises contacting the coating formed by vacuum evaporation with an aqueous solution having a pH within the range of l to 13 containing at least one inorganic chemical oxidizing agent.

9. The method of claim 8 wherein said inorganic chemical oxidizing agent is a permanganate, bichromate or persulfate.

10. The method of claim 8 wherein said aqueous solution has a pH within the range of 2 to 12.

l l. The method of claim 1 wherein the coating which has been subjected to oxidation is then contacted with a chlorinating agent at about room temperature for about 3 seconds to about 30 minutes, said chlorinating agent being selected from the group consisting of chlorine, hydrogen chloride, nitrogen oxychloride, diluted hydrochloric acid or aqua regia.

12. The method of claim 1 wherein said evaporating source consists of at least percent by weight of indie oxide and the remainder being tin oxide or lower oxides of indium.

13. The method of claim 1 wherein said plastic material is a film or sheet.

14. The method of claim rial is a polyester.

15. The method of claim 1 wherein the plastic material is a polyester film, wherein at least 80 mol percent of the recurring units of said polyester consists of polyethylene-2,6-naphthalene dicarboxylate; said film having been biaxially stretched wherein the draw ratio in each of the transverse and longitudinal directions is at least 2.

1 wherein said plastic mate- 

1. A METHOD FOR PREPARING A TRANSPARENT ELECTRICALLY CONDUCTIVE ARTICLE WHICH COMPRISES FORMING A GRAY TO BLACK COLOR COATING OF LOWER OXIDES OF INDIUM ON THE SURFACE OF A PLASTIC MATERIAL BY VACUUM EVAPORATION USING INDIC OXIDE AS AN EVAPORATING SOURCE AT A PRESSURE LOWER THAN 5 X 10**4MMHG WHILE MAINTAINING THE TEMPERATURE OF THE EVAPORATING SOURCE AT 1,300* TO 2,000*C. AND THEREAFTER OXIDIZING SAID COATING UNTIL THE COATING BECOMES TRANSPARENT SO THAT THE TRANSMITTANCE OF 600 NONAMETERS LIGHT THROUGH THE COATING REACHES TO 60 TO 95 PERCENT.
 2. The method of claim 1 wherein the temperature of the evaporating source is 1,450* to 1,800*C.
 3. The method of claim 1 wherein said oxidation treatment comprises heating the coating formed by vacuum evaporation in an atmosphere of air or a gas containing at least 15 percent by volume of oxygen at a temperature of 80* to 250*C. but lower than the heat-resistance temperature of the plastic material.
 4. The method of claim 3 wherein the coating has a thickness of 300 to 2000 A.
 5. The method of claim 1 wherein said oxidation treatment comprises subjecting the coating formed by vacuum evaporation to anodic oxidation in a liquid medium having a specific conductivity of from 10 10 ohm 1 cm 1 to 10 2 ohm 1 cm
 1. 6. The method of claim 5 wherein the liquid medium has a specific conductivity of from 10 9 ohm 1 cm 1 to 10 3 ohm 1 cm
 1. 7. The method of claim 6 wherein the liquid medium is dimethyl sulfoxide or dimethyl formamide.
 8. The method of claim 1 wherein said oxidation treatment comprises contacting the coating formed by vacuum evaporation with an aqueous solution having a pH within the range of 1 to 13 containing at least one inorganic chemical oxidizing agent.
 9. The method of claim 8 wherein said inorganic chemical oxidizing agent is a permanganate, bichromate or persulfate.
 10. The method of claim 8 wherein said aqueous solution has a pH within the range of 2 to
 12. 11. The method of claim 1 wherein the coating which has been subjected to oxidation is then contacted with a chlorinating agent at about room temperature for about 3 seconds to about 30 minutes, said chlorinating agent being selected from the group consisting of chlorine, hydrogen chloride, nitrogen oxychloride, diluted hydrochloric acid or aqua regia.
 12. The method of claim 1 wherein said evaporating source consists of at least 80 percent by weight of indic oxide and the remainder being tin oxide or lower oxides of indium.
 13. The method of claim 1 wherein said plastic material is a film or sheet.
 14. The method of claim 1 wherein said plastic material is a polyester.
 15. The method of claim 1 wherein the plastic material is a polyester film, wherein at least 80 mol percent of the recurring units of said polyester consists of polyethylene-2,6-naphthalene dicarboxylate; said film having been biaxially stretched wherein the draw ratio in each of the transverse and longitudinal directions is at least
 2. 